1. Technical Field
The present invention relates generally to medical devices, and more particularly to medical devices that are magnetically opaque and at least partially implantable into a human or veterinary patient. In preferred embodiments, the invention relates to catheters, cannulae and other medical devices with magnetically opaque contrast agents incorporated along a substantial portion of the body of the device.
2. Background Information
It has become common to treat a variety of medical conditions by introducing an implantable medical device partly or completely into the esophagus, trachea, colon, biliary tract, urinary tract, vascular system or other location within a human or veterinary patient. For example, many treatments of the vascular system entail the introduction of a device such as a stent, a catheter, a balloon, a wire guide, a coil, a cannula, or the like. The device utilized may serve a variety of purposes such as maintaining vessel patency, providing access to a body portion, or introducing therapeutic agents. The implantation of these devices, however, can be damaging. Examples of possible damage include disturbance or injury to vessel walls during navigation, occurrences of clot formation, thrombosis, stenosis or occlusion of the vessel and a general risk of infection. Infection has been controlled by incorporating antibiotics into the medical device, for example, by imbedding an antibiotic powder in the device covered by a permeable layer. In such cases, delivery of the antibiotic occurs by way of dispersion from the device, through the permeable layer and into the surrounding tissue.
The medical industry is increasingly using implantable devices on more patients and in new ways. These devices are becoming an important part of surgeries as they limit incisions and intrusion into the body cavity as well as often having a self contained delivery system for therapeutic agents. A wide range of therapeutic agents can be used and include antibiotics, cancer drugs, chemotherapeutic drugs, numerous disease-specific drugs, genetic material, living cells introduced for transplantation and other biological therapeutic agents. Due to their extensive use, the ability to acquire images of the devices while implanted and/or during implantation is essential. Imaging facilitates precisely locating the device as well as visualizing interactions between the biologic tissue and the medical device. Past surgical procedures using implantable devices often incorporated a camera or scope to visualize the area surrounding the medical device, however such practices are intrusive and increase the chances of tearing or otherwise damaging tissue.
While X-ray and other radio imagery techniques are common, magnetic based imaging is becoming more common and preferred. Magnetic Resonance Imaging (MRI) and Magnetoencephalagraphic (MEG) scans are two common magnetic based imaging procedures. MRI is an often used modality for imaging bone, blood vessels and other living tissue through the detection of magnetic resonance of the biologic matter. While MRI can detect the tissue surrounding a removably positioned device, MRI cannot discern the magnetically translucent devices typically used.
The translucency of the medical device makes it difficult to determine its location relative to the organ structures, tissue and cell clusters of interest. Visualization of medical devices while in use is valuable for numerous reasons, including the ability to monitor and evaluate the efficacy of drug delivery in the area of the device, ability to track the devices movements or the ability to accurately determine relative positions of the device when precision is important to the procedure.
Several techniques have been used in attempts to overcome this problem in diagnostic imaging procedures. Generally, magnetically opaque landmarking devices or fiducial markers are used to denote a particular point on or in the body which is visible to MRI. Magnetically opaque fiducial markers are either internal markers inherent in anatomy or external positional markers temporarily affixed to the body, both creating a reference point. External fiducial markers are generally made from or incorporate a magnetically opaque substance, or contrast agent. Contrast agents are various materials that have a discernable magnetic resonance or magnetic moment which can be induced and hence visualized in magnetic imaging. Common contrast agents include arsenic, gadolinium, solutions or suspensions of tocopherol (vitamin E) and its derivatives, metal ions, salts or chelates such as Nickel Sulfate (NiSO4), among others. For example, vitamin E tablets can be placed externally on various points of the body to denote that point during imaging, producing both anatomical and functional information. While markers can often be used as reference points, many areas of the body contain neither an appropriate internal fiducial marker nor an appropriate place to position an external marker containing contrast agents.
In attempts to visualize or track devices, the medical industry has incorporated a magnetically opaque portion into the device or utilized a second instrument used solely as a reference point.
Examples of MRI-visibility enhanced devices include magnetically opaque paramagnetic instrument tips or MRI-visible micro-coil devices attached to the instrument. Such devices only make a discrete portion of the entire device visible and can be bulky and difficult to efficiently incorporate into the medical device. Another device incorporates a contrast agent-containing fluid in a cavity between two concentric lumens of a catheter, the main purpose of which is to allow the variable pressurization of the catheter by the introduction or removal of liquid, as shown in U.S. Pat. No. 5,897,536. The fluid carrying the contrast agent, however, is only introduced when increased stiffness of the device is desired. Furthermore, the disclosed devices are larger in diameter to create a sufficient fluid cavity for pressurization. A medical device according to U.S. Pat. No. 6,056,700 is capable of taking a biopsy of tissue and then leaving a magnetically opaque marker at the site of biopsy, so that the exact position of the biopsy can be precisely found again. That device does not aid MRI guided procedures, but simply leaves an internal marker behind for future reference. Another medical device is rendered partially MRI visible by the application of dysprosium oxide rings, which are magnetically opaque on the catheter or guidewire. The rings are only located at certain increments of the device and may increase diameter of the device.
Completely separate medical devices have been used for the sole purpose of providing a marker or reference point. For example, U.S. Pat. No. 6,628,982 discloses a separate apparatus carrying an MRI detectable material. Separate devices can be inserted into organs or biological substances that are not usually visable for the sole reason of imaging them, as opposed to performing a functional procedure on the surrounding tissue. Separate marking devices can also be used in addition to medical devices such as catheters and stents, however the increased bulk of instruments intruding on the patient increases the chances of trauma, tearing and infection.
It is desirable to manufacture a medical device which incorporates magnetically opaque material throughout a substantial portion of the device. It is desirable that a device not be significantly larger than similar magnetically lucent devices. It is also desirable to minimize the number of devices used during a procedure by placing the contrast agent on or in the functional medical device. It is also desirable to be able to visualize a large portion of a medical device while at least partially implanted, as opposed to a single point or area of the device. It is likewise desirable to use a minimally sized device which is magnetically opaque such that its travel, location and use can be visually tracked by magnetic imaging modalities.